Continuous HVAC Blower Fan Operation for Optimum Indoor Air Quality InspectAPedia® -
How to improve indoor air quality by continuous blower fan operation in the heating or cooling air handler
Indoor air filtration using the heater or air conditioner blower fan
Why & how to run the blower fan continuously indoors
Warnings about proper blower fan duty cycle and fan speed to avoid blower fan motor failure
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Here we discuss How to how and why to set your air conditioning or heating system blower fan to continuous operation in order to improve indoor air quality by increased and continuous indoor air filtration. (Naturally if there is an IAQ problem source in the building it is essential to also find and correct that condition.
For maximum IAQ improvement in buildings: for the new blower installation
we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which
permits continuous operation if we wish to run the system that way. Running the blower continuously at low speed resulted in continual
air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed.
Why Run an Air Conditioner or Heating Blower Fan Continuously?
There are reasons to leave the fan in the ON position on a heating or cooling system, but we do not recommend that you do this without first asking for advice from your heating and service technician.
If your air conditioner or heating system fan blower unit was designed to permit the fan to run all of the time that's great and you can consider the benefits of continuous fan operation we list just below.
But if your fan motor was not intended for continuous duty and you run the fan that way, you may find the motor fails prematurely.
Here are some possible advantages of continuous blower fan operation
Improved air filtration, lower indoor dust levels - this is particularly true if you have a good air filter system installed. In our forensic lab when we installed a new gas furnace we asked the installer to include a multi speed fan (the fan chooses the needed speed automatically), a fan rated for continuous duty cycle, and we specified that the air filter system include HEPA filtration as well as an electrostatic air cleaner. We also included a cheap disposable filter at the air returns.
This fan plus filter design permitted us to run the blower fan continuously in our lab, regardless of whether or not the heat was running (we don't need central air). We measured airborne dust levels in the building before and after installation of the new system and found that indoor dust levels improved by at least one order of magnitude.
This system was enormously more effective than the dust filtration capacity of any free-standing "air cleaner" found on the popular consumer product market. That's because a warm air central heating system or central air conditioning system blower and duct work move many more cubic feet of air per minute through the filter system than a free-standing room "air cleaner" or "air purifier" can handle.
Increased building comfort: especially if a multi-speed or variable speed fan motor is included in the system design, the continuous movement of air in the building avoids sudden hot or cold blasts when heating or air conditioning turns on; indoor air temperatures tend to be more even.
Here are some disadvantages of continuous air conditioner or heating system blower fan operation
Energy use: A downside of continuous blower fan operation is higher electrical costs. On the other hand, if we needed to reduce building dust and allergen levels and/or increase comfort levels, this approach might avoid wasting money on other attempts at air cleaning that are expensive and less effective. More careful study is needed to really evaluate the claim that continuous blower fan operation uses more overall energy. In some cases there may be a net energy savings, depending on how hard the heating or cooling system has to work to satisfy the thermostat (and occupants) during "on" cycles.
Equipment life: if the blower motor is not rated for continuous duty operation, it may need replacement sooner when continuous fan operation is used. In this case be sure the new blower is rated for continuous duty.
Air Filtration Suggestions
See these articles about how to use air filters, where to buy them, and how to turn on and off HVAC blower fans
Other Switches on a Room Thermostat - how to set the HVAC blower fan to continuous operation, and what to do if the blower fan won't turn on or won't turn off
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Wikipedia provided background information about the definition of HEPA and airborne particle interception.
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